In recent years, in accordance with an increase in communication traffic, there has been a demand to increase the communication capacity in communication systems such as point-to-point. To meet this demand, it is known to use a communication system that uses a polarized wave shared antenna capable of transmitting or receiving a polarization multiplexed signal including two polarization wave signals having polarization planes perpendicular to each other in order to perform communication by the polarization multiplexed signal. According to this communication system, information can be carried on each of the polarization wave signals, whereby it is possible to double the communication capacity compared to the case in which the polarization multiplexed signal is not used.
A method of transmitting or receiving polarization multiplexed signals by a parabola antenna is already known. Since the parabola antenna has a relatively large thickness and affects wind loads or landscapes, however, a planar antenna has been introduced.
As an example of a polarized wave shared planar antenna, a planar antenna having a structure in which conductors, which are antenna elements, are connected by microstriplines (power feed lines) is disclosed (Patent Literature 1).
A polarized wave shared square opening antenna capable of efficiently separating or combining a vertical polarization wave and a horizontal polarization wave when receiving a polarization multiplexed signal by a square opening or transmitting a polarization multiplexed signal from the square opening is disclosed (Patent Literature 2).
Another antenna apparatus capable of attenuating, when a transmission is performed using rectangular waveguides through which higher-order modes can be propagated, the higher-order modes that can be propagated is disclosed (Patent Literature 3).